The effect of photoacclimation on the photosynthetic physiology ofUlva curvataandUlva rotundata(Ulvales, Chlorophyta)

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Abstract
The effects of photoacclimation on growth, photosynthesis, pigment content and elemental composition of Ulva curvata and Ulva rotundata, which grow together in eutrophic areas of southern Spain, were investigated. Cultures were grown for 6 days at different photon fluence rates (PFR) ranging from darkness up to 200 µmolm-2 s-1 under nutrient-sufficient conditions (artificial seawater supplemented with ammonium and phosphate). Growth rates were not light-saturated (up to 200 µmolm-2s-1), reaching a value close to 0·2d-1. Growth rates based on mass, area or C content were equivalent, except in darkness and very low light levels (2 µmolm-2s-1), where thallus expansion occurred by diluting internal biomass and C. Chlorophyll and absorptance showed a bell-shaped PFR-response curve, with maxima at 30–60 µmolm-2s-1 and lower values at light saturation and under light-limiting conditions. Although net NH4 + incorporation was not affected by growth-PFR, there was inefficient assimilation of N at low light levels, which may restrict chlorophyll, protein and membrane synthesis. The light-saturated photosynthesis rate (P max) displayed a bell-shaped PFR-response curve, when expressed on an area basis, whereas it was saturated from 11µmolm-2s-1 when scaled to internal N. This suggests that ribulose-1,5-bisphosphate carboxylase/oxygenase activity could be the rate-limiting step underlying P max for high-light-acclimated algae, whereas electron transport chain elements may limit P max under light-limiting conditions. P max and tissue C were always higher for U. curvata. Dark respiration rates were positively correlated with growth rates, and photon yield of net growth declined with increasing growth-PFR. The results are also discussed in relation to cell size, since U. curvata cells are smaller than those of U. rotundata.